1. Field of the Invention
The present invention relates generally to the control of the temperature of the web in heat-set web printing. More particularly, this invention relates to the apparatus and process for controlling the web temperature by the use of "chill" rolls.
2. Description of the Prior Art
In heat-set web printing, after the ink is applied to the paper, the web leaves the hot air dryer at a temperature between 230.degree. F. and 280.degree. F. At this stage the ink is still soft and requires immediate cooling before the paper can be processed further. Typically, the cooling of the web has been accomplished by the use of what are called chill rolls; the hot web is caused to traverse a series of two to six hollow steel rollers that are cooled with circulating chilled water.
During normal operation of the printing apparatus, frequent interruptions will occur in the continuity of the web movement. This may result from the expected end of a job or change of printing plates, or it may result from the unexpected break of the web. Regardless of the cause, when such a stoppage occurs the prior art systems often allow the web temperature to fall below the dew point. This causes condensation (often referred to as "sweating") to accumulate on the surface of the rolls, saturating the paper web. The resulting web is weakened and likely to break upon restart of the line.
Attempts to control this condensation or sweating of the chill rolls has included control of the flow of chilled water through the chill rolls during web stoppage. However, the thermal mass of the water already in the chill rolls, as well as the thermal mass of the chill rolls themselves, will still cause the temperature of the surface of the chill rolls to drop below the condensation temperature. To remedy that problem, electric heaters have been used in the prior art during web stoppage to heat the circulating chill water above the condensation temperature. But that system has had only limited success since it takes several minutes to warm the water and the chill rolls to a temperature that is high enough to stop condensation on the chill rolls. Alternatively, a storage tank filled with hot water has been used to flush the chill rolls during the press shutdown. While this system has proved to be faster than the electric heater for a single press shutdown, the tank will inevitably cool too much during frequent stops to be effective.